1. Field of the Invention
The present invention relates to a semiconductive roller useful as a developing roller, and an image forming apparatus comprising the same such as copier, printer or facsimile apparatus.
2. Description of the Prior Art
A semiconductive roller is composed by covering the outer circumference of a conductive shaft body with an elastic layer, and is presented for various applications, and one of such applications is the image forming apparatus such as an electrophotographic printer.
The image forming apparatus of this type comprises a photoreceptor drum 10 as shown in FIG. 8, and this photoreceptor drum is surrounded by a charging roller 11 for charging the drum at a constant potential, an LED array 12 as an exposure unit for forming a latent image on the drum surface, a developing roller 13 for applying toner to the latent image so as to obtain a visible toner image, a transfer roller 15 for transferring the toner image of the photoreceptor drum material on a recording paper 14, a cleaning roller 16, and others, and further a toner conveying roller 17, a friction electrifying blade 18, a toner agitator 19 and others are disposed in the vicinity thereof. The image forming apparatus having such configuration functions to support the friction electrified toner on the outer circumference of the developing roller 13 in a thin layer state, and develop and visualize the formed electrostatic latent image.
The semiconductive roller is used as developing roller 13 or the like for such image forming apparatus, and in this case, aside from mechanical properties, it is required to have various properties including conductivity, environmental resistance, low hardness, and friction electrifying characteristic. Accordingly, the semiconductive roller is composed of urethane rubber, NBR, EPDM rubber, silicone rubber or similar material, and by adding and blending conductivity donor such as electroconductive substance or ion conductive substance.
When the semiconductive roller is used as the developing roller 13, it rotates while keeping contact with the photoreceptor drum 10 or friction electrifying blade 18, and it is important to contact closely with them more uniformly and in a wider nip. Therefore, the developing roller 13 is required to have a low hardness, instead of high hardness. Being low hardness is effective from the viewpoint of driving torque and damage to the toner.
Hitherto, to lower the hardness of the semiconductive roller, either one of the following methods has been employed: (1) a method of lowering the hardness by adding process oil, softening agent or other liquid, and forming a protective layer of urethane resin or nylon resin on the surface for prevention of bleeding of the liquid, and (2) a method of covering the outer circumference of the rubber foamed body with unfoamed rubber.
In the method of (1), however, by using the liquid softening agent, the environmental resistance is impaired, and the electrical properties and volume are changed by change in temperature or humidity. Furthermore, prevention of bleeding is not satisfactory, and other members may be contaminated.
In the method of (2), the outer circumference of the rubber foamed body is covered with unfoamed rubber, but in this case since the covering layer needs a certain thickness in order to lessen the effect of cells of the lower layer, it is contrary to reduction of hardness. In addition, since the conductive layer is a foamed body, the electrical characteristics are not stable, and it is very difficult to polish and finish the roller surface at high dimensional precision after covering.
The present invention is devised in the light of the above problems, and it is hence an object thereof to present a semiconductive roller of low hardness, excellent in environmental resistance, superior in resistance distribution and polishing processability, and capable of obtaining excellent images in a wide range of conditions, and an image forming apparatus.
The present inventor intensively studied in order to solve the problems, and came to conclusion that the object may be achieved by composing the semiconductive roller in a multi-layer structure by forming an elastic layer in an unfoamed body of addition curing type liquid silicone rubber, and combining this unfoamed body with various outer layers.
In accordance with a first aspect of the invention, a semiconductive roller comprises an elastic layer formed on the outer circumference of a conductive shaft body, wherein the elastic layer is an unfoamed body having the addition curing type liquid silicone rubber cured as a base polymer, and the ASKER C hardness of this unfoamed body is 10xc2x0 to 50xc2x0.
In accordance with a second aspect of the invention, an image forming apparatus comprises a semiconductive roller comprising an elastic layer formed on the outer circumference of a conductive shaft body, wherein the elastic layer is an unfoamed body having addition curing type liquid silicone rubber cured as a base polymer, and the ASCA C hardness of this unfoamed body is 10xc2x0 to 50xc2x0.
Preferably, the outer circumference of the elastic layer is preferably covered with a semiconductive unfoamed rubber elastic layer. Also, the outer circumference of the elastic layer should be provided with either a surface layer of a crosslinking resin or a surface-treated layer. Further, the outer circumference of the elastic layer is preferably covered with a semiconductive unfoamed rubber elastic layer, and the outer circumference of this unfoamed rubber elastic layer is preferably provided with either a surface layer of a crosslinking resin or a surface-treated layer.